There has been known an image drawing apparatus employing an anti-aliasing technique for removing an aliasing which appears as a stage-shaped portion at the boundary of a polygon when this polygon is displayed on a display apparatus or a screen. This anti-aliasing technique is a technique for providing an intermediate color to a boundary portion of a drawing thereby to make a luminance difference at the boundary portion not easily noticeable.
As representative anti-aliasing techniques conventionally available, there are an over-sampling method, an area rate method and a two-time drawing method. The over-sampling method is a technique in which a polygon is drawn in an image space larger than display pixels, and a plurality of pixels are combined to form in one display pixel in compression at the time of a display.
The area rate method is a technique of calculating a linear line forming each side of a polygon in units smaller than pixels, calculating a rate of an area primarily occupied by the polygon in boundary pixels, and carrying out a semi-transparent processing in pixels already drawn according to the rate calculated.
The two-time drawing method is a technique of drawing a linear line based on an aliasing processing along each side of a polygon separate from a drawing of the polygon, by using a linear line drawing apparatus for carrying out an anti-aliasing processing.
According to the prior-art anti-aliasing technique, however, there are problems that a large memory capacity is required and that a drawing processing is delayed. In other words, when the over-sampling method is used, it is necessary to hold a larger memory area in the drawing space than a memory area primarily required for the display. Further, the load of processing increases by the portion of the drawing area expanded, which results in a processing delay.
When the area rate method is used, it is necessary to calculate an area for each pixel forming a boundary portion, and therefore, it takes time for a drawing processing.
Further, when the two-time drawing method is used, it is necessary to carry out a linear line drawing processing independent of a polygonal drawing processing, in addition to the polygonal drawing processing primarily required. Therefore, it is not possible to make the most of a general advantage of a memory that a processing can be executed at a higher speed in a near memory area. As a result, a processing delay is generated.
As explained above, according to the prior-art anti-aliasing processing, there are severe problems from the viewpoint of memory capacity and processing delay. Therefore, it has been an extremely important subject to find out how to achieve at high speed and in a small memory capacity, the anti-aliasing processing for making look smooth a stage-shaped boundary in pixel unit appearing on each side of a polygon.